The present invention relates in general to circuits for converting sampled points along an analog waveform to stored digital data points and in particular to such a circuit as will minimize the number of stored sample points required to accurately characterize the analog signal.
In a number of applications, including storage type oscilloscopes, an analog waveform is sampled by an analog-to-digital (A/D) converter at regular intervals and converted to a series of digitally encoded signals representing the magnitude of the sampled analog waveform. The digitally encoded signals are then stored in a digital memory. If the stored points are later applied in proper order to the input of a digital-to-analog (D/A) converter at the original sampling frequency, the D/A converter output will recreate the original waveform.
The accuracy of waveform recreation depends primarily on the sampling frequency. A rapidly varying waveform requires a higher sampling frequency than a slowly varying waveform for similar reproduction accuracy and therefore requires a larger memory for data storage, if all the sampled points are stored. Sampled waveforms sometimes vary in an irregular fashion with one part varying slowly, or not at all, and with another part of the waveform varying rapidly. This is typical of a "burst" type waveform. A sampling rate adjusted for accurate characterization of the rapidly varying portion of the waveform is much higher than necessary for accurate characterization of the slowly or nonvarying portion of the waveform and would produce many more points during the slowly varying portion than needed for accurate waveform reproduction. These additional data points use up memory space.
In the prior art, memory requirements have been reduced by varying the sampling rate, using a faster sampling rate when an analog waveform varies rapidly and a slower sampling rate when the waveform varies slowly. In Japanese Laid Open Pat. No. 17598/83 (filed July 24, 1981 and laid open Feb. 1, 1983), successive sample points are continuously compared. If they differ by more than a set amount, the sampling rate is increased to a higher rate. When the next sample point is stored, a flag is stored with the data to indicate that the sampling period for that data was shorter. Should any successive samples differ by less than the set amount, the sampling rate is reduced to a lower rate. Thus slowly varying portions of an analog waveform are sampled at one rate while more rapidly varying portions of the signal are sampled at a second and higher rate.
For a given level of accuracy, the number of stored sampled points required to characterize an irregularly varying analog waveform is thus reduced over the number required by the traditional, constant sampling rate, approach. However, the amount of memory reduction is limited when only two sampling rates are permissible. Also when the circuit is operating at the lower sampling rate, a higher frequency signal burst could be missed, leading to some inaccuracy.
Therefore, what is needed is a means to minimize the amount of memory required to store an accurate digital representation of a sampled analog waveform.